BERLIN (AP) — Berlin police said 13 journalists were detained Saturday while reporting on a protest by environmentalist against the construction of a highway in the German capital.

The journalists were covering an early-morning blockade of the construction site of the A100 Autobahn by activists from the group Sand im Getriebe, or Sand in the Gears.

“They entered the site along with the activists, so we treated them as participants in the protest," police spokesperson Martin Halweg said.

They are now being investigated on suspicion of trespassing on the construction site, which is operated by the publicly owned company Autobahn GmbH, Halweg said.

Joerg Reichelt, the chief executive of the journalists' union DJU in Berlin and Brandenburg state, accused police of preventing reporters from doing their work. Reichelt was one of the journalists detained and facing possible trespassing charges.

Some 300 people took part in the demonstration. Police physically removed several protesters from the site before the remainder agreed to submit to photo identification as a condition of being allowed to leave, Halweg said.

Environmental groups say the building of new highways in Berlin and other parts of the country undermines Germany's efforts to curb greenhouse gas emissions. They argue that billion of euros (dollars) spent on roads could be used instead to pay for climate-friendly transportation infrastructure such as railways.